In many manufacturing processes, it is often necessary to align two objects, such as two layers in which one layer will overlie the other. For example, in semiconductor manufacturing, successive masks which define the semiconductor structure are placed over the semiconductor substrate at various stages in the manufacturing process. Each successive mask must precisely align with the position of a previous mask on the semiconductor substrate.
As another example, in color printing, an original color image is scanned to separate the original image into several component colors, each of which component color will be separately printed in alignment with the layers above and below. Therefore, the color printing plates must be manufactured so that each of the separate color images are printed in precise alignment or registration with each other.
Specifically, an original input image is processed to produce four screened images: one for each of the printing inks of cyan, magenta, yellow and black. The four screened images are then used to burn printing plates needed in a four color printing reproduction process. In burning the plate, the image negative is affixed to a transparent acetate sheet with holes punched. The acetate sheet is then placed on the plate burning machine, which has metal tabs that fit into the holes in the acetate sheet. Each of the acetate sheets must have negatives at exactly the same position relative to the holes in order to insure that the plates have images in the same place. Accurate placement of the negatives on the acetate sheets is even more important if each plate is to be burned with more than one negative each.
To accurately place the negative on the acetate sheet, register marks are used. The first negative, typically corresponding to the black image plane, is placed on a light table that has metal tabs to fit the holes in the acetate sheet. The location of the first negative on the first acetate sheet is not too critical; a typical tolerance is on the order of 50 mils. The first negative placement determines the general location of the color image on the printed page.
The next step is to put another acetate sheet over the first sheet, and place a second negative, say the negative corresponding to the magenta image plane on the second acetate sheet in precise relation to the first negative on the first acetate sheet. To facilitate alignment, image negatives have register marks which are not part of the image and are later removed. The operator examines the register marks, usually with the aid of a magnifier. By close examination or the register marks, the operator determines the position of the second negative on the second acetate sheet for which the two register marks are exactly overlaid. The second acetate sheet carrying the aligned second image negative (magenta) is then removed. The process is repeated for a third acetate sheet and a third negative, say for cyan, and again for yellow. The acetate sheets are then used as indicated above to burn printing plates. The quality of color printing is dependent upon the accuracy of alignment of the four color images in the overall plate burning and printing process.
Automated processes for the automatic alignment of register marks are also known. The composite image of the two register marks is captured in a memory, the alignment error is measured, and one plate is moved in a direction so as to reduce the measured error. When the measured error is minimized, the register marks are aligned.
There are many known prior art register marks used to align two overlying transparent layers. One type of conventional register mark consists of crosshairs, i.e. a plus sign. This form of register mark has two disadvantages. First, a crosshairs register mark requires close examination, usually with the aid of a magnifier, to determine exact alignment. Second, when the two planes are not in good register, it is very difficult to determine from the non-aligned crosshair composite image which direction to move the top plane. Generally, trial and error movements indicate to the observer which direction to move the top plane. The need for close inspection and trial movements makes automated adjustment complex and expensive, in both the optical system and in the mechanical control system.